During a production process of metal can bodies such as seamless can bodies produced by drawing with ironing (hereinafter simply referred to as “can body”), pinholes in the form of holes or cracks may be created in a body part or the like of the can body. Commonly, presence or absence of such pinholes is determined by using a pinhole inspection apparatus in an inspection process of can bodies.
The pinhole inspection apparatus shown in Patent Document 1 is configured, as shown in FIG. 10, with an inspection turret that holds can bodies to be inspected on the right side, and an optical apparatus for inspecting the interior of can bodies on the left side. A piston-like sealing part is provided with a sealing ring plate fixedly attached thereto for sealing a sliding contact surface with a sliding ring plate by making sliding contact therewith. The piston-like sealing member is mounted on a distal end face of a first frame on the side facing the sliding ring plate for making the sliding contact. During pinhole inspection, the piston-like sealing part is pressed against the sliding ring plate rotating at high speed with air pressure, to enhance the sealing effect at the sliding contact surface, and to prevent ambient light, which consists of outside light and light from a light source, from reaching a photodetector side through the sliding contact surface.
Since the sealing ring plate is kept in pressure contact with the rapidly rotating sliding ring plate during pinhole inspection as noted above, ambient light normally does not reach the photodetector through the sliding contact surface between the sealing ring plate and the sliding ring plate. However, there was a possibility that ambient light may reach the photodetector through a gap that may be created instantaneously between the sealing ring plate and the sliding ring plate due to various factors such as some form of external force applied thereto, or the surface conditions of these plates, and good products may be erroneously determined as defective.
Patent Document 2 describes how to reduce stray light entering into an opening in a base member through a gap between a moving member and the base member by forming a V groove with inclined surfaces of different angles in a surface of one of a fixed disc and a rotating disc spaced apart from each other.
Patent Document 3 describes a light shield part made up of an annular, circumferentially oriented groove in a surface of a rotating disc opposite a movable plate, with a distal end portion of a fixed disc being located inside this circumferential groove. The light shield part allegedly prevents entrance of light emitted from a light source into a light guide path through between the fixed disc and the rotating disc.
In Patent Document 2, although a cover-like member is attached to peripheral and inner edge parts of the moving member, the fixed disc and the rotating disc are spaced apart from each other as a precondition. It is therefore assumed that, if a high lumen light source is employed for increasing pinhole inspection sensitivity, a significant amount of stray light would reach inside the opening where a photodetector is located due to diffused reflection on inner surfaces of the fixed disc and rotating disc.
The light shield part described in Patent Document 3 employs a complex structure wherein the fixed disc fits into the opposing circumferential groove, which makes position adjustment difficult. While the document discloses provision of a trap on the surface of the fixed disc to return the light to where it comes from, it does not fully explain the advantageous effects thereof and therefore fails to offer a complete solution to the problems mentioned above.
The diameter of pinholes to be inspected by a conventional pinhole inspection apparatus for can bodies is about 20 μm, which is determined by the performance of the photodetector. Pinholes with an even smaller diameter may not pass an enough amount of light into the can body to be detectable. A possible approach would be to secure a sufficient amount of light for enabling detection by the photodetector by using a light source with higher brightness than conventional light sources. However, the conventional apparatus configuration does not have sufficient countermeasures against ambient light, and the increase in the light amount could lead to an increased erroneous detection rate.